The present invention relates generally to a method and apparatus for spectroscopic analysis of chemical compounds that have been separated by a gas chromatograph and suspended in a frozen matrix isolated state, and specifically to an arrangement for collecting, entrapping and displaying the separated compounds in a state of matrix isolation for spectroscopic analysis.
Conventionally, gas chromatographs been widely used in conjunction with various spectroscopic instruments, for example, mass spectrometers, for analysis of chemical compounds. In such systems, gas chromatographs, which are known instruments for separating mixtures of chemical compounds, separate the samples into their respective individual compounds. The separated compounds are then analyzed by the mass spectrometers to determine the identity of each compound.
While conventional mass spectrometers may be sufficient for supplying approximate analyses of the separated compounds, they are inadequate for distinguishing between compounds having molecules that are very similar, i.e., isomers. The problem is compounded when the quantity of a sample being analyzed is small. In addition, the mass spectrometer does not offer any means for preserving the samples for further analysis at a later time. For these reasons, the conventional gas chromatograph/mass spectrometer system is unsuited for identification of minute amounts of compounds requiring a high degree of specificity, such as when exact isomer identification is required.
To address the above-described shortcomings of the gas chromatograph/mass spectrometer system, a matrix isolation infrared spectroscopic technique has been developed to be used in conjunction with the gas chromatograph. In this system, a chemical sample is gas chromatographically separated, and mixed with an inert substance, such as argon or krypton gas. Both the sample and the inert gas are then deposited onto a smooth mirrored surface that is cryogenically cooled to about 10.degree. to 20.degree. K., which results in the molecules of each compound being entrapped within a frozen matrix of solidified inert gas. The infrared spectrum of each molecule is then measured for high resolution identification of each compound in the sample. In this manner, an accurate analysis of the chemical compounds is achieved with more certainty than from conventional mass spectrometers, particularly when it is important to distinguish among compounds that have very similar molecules and when small quantities of samples are being tested. An added advantage of this technique is that it permits retention of the sample in a suspended state over an extended period of time to allow a confirming analysis at a later time.
A known apparatus for incorporating the matrix isolation technique with a gas chromatograph is disclosed in U.S. Pat. No. 4,688,936 to the present inventor, which is incorporated by reference herein. That apparatus includes a sample collection vacuum chamber. Provided within the vacuum chamber is a specular carousel having a polished surface, upon which the gas sample from the gas chromatograph is deposited by a delivery nozzle. The specular carousel is configured to rotate in a helical pattern so that there is a continuous surface for the sample to be deposited. Coupled to the vacuum chamber is a vacuum pump for creating a high vacuum. The expander module of a closed cycle refrigerator, which is also coupled to the vacuum chamber, is included for supporting and cryogenically cooling the specular carousel. When a beam of infrared (IR) light enters the vacuum chamber, it is adjustably directed onto the sample deposited on the carousel surface by a first mirror. A second mirror, positioned adjacent to the first mirror, receives the IR beam reflected from the carousel surface and aims the IR beam to an infrared light detector.
One disadvantage of the system disclosed in U.S. Pat. No. 4,688,936 is that the specular carousel is bolted to the expander module and its rotation is derived by rotating the entire expander module. This is an inefficient method of rotating the specular carousel, because the only function of the expander module is to cool the specular carousel, and therefore, configuring the expander module to also rotate the carousel unnecessarily complicates the cooling system and its manufacture. Moreover, the rotation of the carousel via the expander module, rather than rotating it independently, requires a much larger scale of motion, because the motion is transferred to a number of different components before actually reaching the carousel. Consequently, a precise rotation of the carousel is not achieved, and the sample is not deposited in a consistent helical pattern to be exposed by the IR beam.
A sample block that rotates independently of a cooling expander is disclosed in U.S. Pat. No. 4,158,772 to the present inventor. However, it has been found that the arrangement disclosed in that patent results in poor heat transfer between the cooling expander and the sample block. This is because the heat transfer is conducted through threads on which the sample block rotates and translates, resulting in limited contact between the sample block and the threads. In addition, the sample block of this patent is octagonally shaped, which reduces the availability of the sample collecting surfaces.
Another disadvantage of the above-identified prior art matrix isolation analysis systems is that the point of sample deposition on the carousel is 180 degrees away from the point where spectroscopic analysis is conducted. As a result, the recording of the IR beam for analysis is a "post-run" operation, in contradistinction to a "real time" operation. This drawback results in unwanted delay in the analysis.
Still another disadvantage of the prior art matrix isolation analysis systems is that the delivery nozzle is stationary, which results in unwanted excess sample being deposited on the sample block due to weeping. Another disadvantage of the systems having the stationary delivery nozzle is that unwanted solvent from the sample is discharged onto the sample block, i.e., an upstream valving operation for eliminating the solvent must be performed on the sample block. A displaceable delivery nozzle is disclosed in U.S. Pat. No. 4,594,226 to the present inventor. However that delivery nozzle is not displaceable in a manner which would prevent unwanted excess sample from contaminating the carousel surface.
Yet another disadvantage of the prior art matrix isolation analysis system is that the first and the second mirrors are two separate pieces, and require adjustments relative to each other. In addition, the adjustment assembly on which the two mirrors are mounted for adjusting the two mirrors relative to the carousel surface only allows two dimensional adjustments.
Thus, it is a first object of the present invention to provide an apparatus for spectroscopic analysis of chemical compounds having a deposition surface that moves independently of the cryogenic cooling system, yet provides efficient heat transfer between the deposition surface and the cooling system.
Another object of the present invention is to provide an apparatus for spectroscopic analysis of chemical compounds having a sample deposition point at close proximity to the point of spectroscopic analysis to enable "real time" examination of the sample.
Yet another object of the present invention is to provide an apparatus for spectroscopic analysis of chemical compounds having a sample delivery nozzle that can be rapidly displaced from the deposition surface to prevent excess sample and unwanted solvent component of the sample from contaminating the deposition surface.
Still another object of the present invention is to provide an apparatus for spectroscopic analysis of chemical compounds wherein the two mirrors for focusing and redirecting the IR beam are fabricated in a single block, and an adjustment assembly for adjusting the mirrors relative to deposition surface allows for three dimensional adjustments.